import java.util.Scanner;
public class Seventh {
	public static void main(String[] args) {
	Scanner SC=new Scanner(System.in);
	System.out.println("Please enter the value of n:");
	int n =SC.nextInt();
	System.out.println(getmath( n));
	}
	public static  int getmath(int n) {
	int sum =1;
	if(n < 0){
	System.out.println("Negative numbers have no class!");
	}else if (n==1||n==0){
	System.out.println(n +"class for 1");
	}else{
	for(int i =0;i<n;i++){
		sum +=i;
		}return sum;
	}return sum;
	}
}